Voltage sag
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A voltage sag (U.S. English) or voltage dip[1] (British English) is a short duration reduction in rms voltage which can be caused by a short circuit, overload or starting of electric motors.[2] A voltage sag happens when the rms voltage decreases between 10 and 90 percent of nominal voltage for one-half cycle to one minute.[2][3] Some references defines the duration of a sag for a period of 0.5 cycle to a few seconds,[4][5] and longer duration of low voltage would be called a “sustained sag”.[4]
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Related notions[edit]
The term “sag” should not be confused with brownout which is the reduction of voltage for minutes or hours.[6]
The term “transient” as used in power quality is an umbrella term and can refer to sags, but also to swells, dropouts, etc.[7]
Swell[edit]
Voltage swell is the opposite of voltage sag. Voltage swell, which is a momentary increase in voltage, happens when a heavy load turns off in a power system.[8]